Tennis racket and method for manufacturing the same

ABSTRACT

A tennis racket includes a frame that surrounds a hitting face for hitting a ball, wherein a foam material is provided inside two side portions of the frame on opposite sides of a predetermined area of the hitting face, and the two side portions of the frame have a strength that is higher than a strength of another portion of the frame.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority upon Japanese Patent ApplicationNo. 2012-40538 filed on Feb. 27, 2012, which is herein incorporated byreference.

BACKGROUND

1. Technical Field

The present invention relates to a tennis racket and a method formanufacturing a tennis racket.

2. Related Art

A tennis racket includes a frame that surrounds a hitting face forhitting a ball. There has also been proposed a tennis racket in whichthe frame is formed hollow with the entire hollow portion is uniformlyfilled with foam material (e.g., see Japanese Patent ApplicationLaid-Open Publication No. 6-105923).

The area of the hitting face that is frequently used differs from playerto player in some cases. For example, top players (advanced players)tend to frequently use the area of the hitting face at the tip side(side opposite to the grip portion) than the substantially centralportion (sweet spot). For this reason, there is demand for an increasein particularly rigidity (strength) and vibration absorption in theportions of the frame that correspond to the area that is frequentlyused.

Incidentally, since a foam material is formed uniformly inside the frameof tennis rackets such as those described above, it has been difficultto increase rigidity and vibration absorption at specific positions inthe frame. Increasing rigidity by providing foam material to the entireframe leads to a problem of an increase in repulsive force (due to adecrease in frame flexure) and a decline in control when hitting theball (ball control), and also leads to a problem of difficulty in weightreduction.

SUMMARY OF THE INVENTION

The present invention has been made in view of these circumstances, andan object thereof is to provide a tennis racket that allows improvementin rigidity and vibration absorption at a desired position and allowsimprovement in ball control and reduction in weight, as well as a methodfor manufacturing the same.

A tennis racket according to a main invention for achieving theaforementioned object is a tennis racket including a frame thatsurrounds a hitting face for hitting a ball, wherein a foam material isprovided inside two side portions of the frame on opposite sides of apredetermined area of the hitting face, and the two side portions of theframe have a strength that is higher than a strength of another portionof the frame.

According to the tennis racket of the present invention, rigidity andvibration absorption at a desired position in the frame can be improved,and ball control can be improved and the weight thereof can be reducedas well.

Other features of the present invention will become clear from theexplanation in the present specification and the description of theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view of a tennis racket 1 according to an embodimentof the present invention;

FIG. 2 is a side view of the tennis racket 1 according to the embodimentof the present invention;

FIG. 3A is a schematic diagram showing a method for manufacturing thetennis racket 1;

FIG. 3B is another schematic diagram showing the method formanufacturing the tennis racket 1;

FIG. 3C is another schematic diagram showing the method formanufacturing the tennis racket 1;

FIG. 3D is another schematic diagram showing the method formanufacturing the tennis racket 1;

FIG. 4A is a cross-sectional view taken along A-A in FIG. 3B;

FIG. 4B is a cross-sectional view taken along B-B in FIG. 3B;

FIG. 5 is a schematic diagram showing an aspect of a hardness testperformed with the foam material alone;

FIG. 6 is a schematic diagram showing an aspect of a test regarding theamount of cross-section deformation of the racket;

FIG. 7 is a schematic diagram showing an aspect of a racket bendingtest; and

FIG. 8 is an illustrative diagram of a modified example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Summary of Disclosure

At least the following matters will be made clear by the explanation inthe present specification and the description of the accompanyingdrawings.

Specifically, the tennis racket that will be made clear is a tennisracket including a frame that surrounds a hitting face for hitting aball, wherein a foam material is provided inside two side portions ofthe frame on opposite sides of a predetermined area of the hitting face,and the two side portions of the frame have a strength that is higherthan a strength of another portion of the frame.

According to such tennis racket, rigidity and vibration absorption at adesired position in the frame can be improved, and ball control can beimproved and the weight thereof can be reduced as well.

In the above tennis racket, a foam material for filling that isdifferent from the foam material is provided inside the another portionof the frame. Also, the foam material is provided inside the anotherportion of the frame, and an amount of the foam material in the two sideportions is different from an amount of the foam material in the anotherportion.

According to such tennis racket, a hollow portion can be prevented frombeing made to the frame, and foreign particles can be prevented fromentering into the frame.

In the above tennis racket, it is desirable that the frame has a frameportion that surrounds the hitting face on one end side, and has a gripportion on another end side, and the predetermined area is an area onthe one end side than substantially a middle of the hitting face.

According to such tennis racket, optimum hitting characteristics (e.g.,control) can be obtained when used by a player who often uses one endside of the hitting face (e.g., a top player).

In the above tennis racket, it is desirable that when a position of theone end of the frame portion is set at 0 degrees relative to a center ofthe hitting face, the foam material is provided in a range of 20 degreesto 60 degrees of the frame portion.

According to such tennis racket, it is possible to increase rigidity andvibration absorption when hitting the ball with one end side of thehitting face.

Also, a method for manufacturing a tennis racket including a framehaving a first strength and a second strength that is higher than thefirst strength, including the steps of arranging a foam material on aresin sheet in an area that corresponds to the second strength, forminga sheet tube by wrapping the resin sheet, on which the foam material isarranged, into a cylindrical shape, and forming a frame in which theresin sheet and the foam material are integrally molded, by bending thesheet cylinder to be set in a die, and then causing the foam material toundergo foaming.

Embodiments Example of Racket Configuration

A configuration example of a tennis racket 1 according to the presentinvention will be described below with reference to FIGS. 1 and 2. FIG.1 is a front view of the tennis racket 1 according to the presentembodiment. FIG. 2 is a side view of the tennis racket 1 according tothe present embodiment. Note that the upper side (with respect to thefigure) of the tennis racket 1 shown in FIGS. 1 and 2 is called the tipside, and the lower side is called the base side.

As shown in FIG. 1, the tennis racket 1 of the present embodimentincludes a frame 10 that is configured by a face portion 12(corresponding to the frame portion), a shaft portion 13, and a gripportion 14.

The face portion 12 is provided in the top portion of the tennis racket1 and is formed in a substantially elliptical shape. Insertion holes(not shown) for inserting a string are provided, in an innercircumferential portion and an outer circumferential portion of the faceportion 12 (and a later-described yoke portion 16), so as to communicatethe inner and outer portions, and therefore a string that is insertedinto the insertion hole at the outer circumferential portion can betaken out from the insertion hole at the inner circumferential portion.A plurality of these insertion holes (not shown) are provided in a linealong the circumferential direction of the face portion 12. When astring is strung between the insertion holes, the string segments crosseach other in a grid form within the face portion 12. A substantiallyelliptical hitting face (face for hitting a ball) is thus formed withinthe face portion 12. Also, a sweet spot S shown in FIG. 1 is asubstantially middle area of the hitting face that includes the center Cof the hitting face, and the size of this area is dependent on the shapeof the hitting face (in other words, the shape of the face portion 12seen from the front).

The shaft portion 13 is a portion that connects the face portion 12 andthe grip portion 14, and is formed so as to branch out into two from thegrip portion 14 toward the face portion 12. Also, the yoke portion 16that configures a portion of the hitting face is provided at theboundary between the face portion 12 and the shaft portion 13.

The grip portion 14 is the portion that is gripped by the player, and ispositioned in the base end portion of the tennis racket 1. Normally,grip tape 20 is wound around the grip portion 14 of the tennis racket 1.The grip tape 20 absorbs the shock generated when hitting a ball, andalso improves the grip.

Note that a fiber-reinforced resin, which is obtained by impregnatingfiber with resin such as mainly carbon fiber, glass fiber, organicfiber, ceramic fiber, or the like, is used to form the frame 10 (theface portion 12, the shaft portion 13, and the grip portion 14) of thetennis racket 1. In the present embodiment, the frame 10 uses athermosetting fiber-reinforced resin whose reinforcing fiber is carbonfiber, and as will be described later, the frame 10 is formed by rollingup the fiber-reinforced resin in a sheet form (a carbon sheet) so as toform a hollow bar body, bending the bar body into the shape of thetennis racket 1, and then fitting the bar body into a predetermined dieand performing heat and pressure molding.

The frame 10 of the present embodiment is also provided with a foammaterial 30 in the areas indicated by hatching (referred to hereinafteras hatched portions) in the face portion 12 that surrounds the hittingface. As shown in FIG. 1, the portions where the foam material 30 isprovided are areas on respective sides (two side portions) of the faceportion 12 that are on opposite sides of a predetermined area of thehitting face on the tip side than the sweet spot S. More specifically,assuming that the tip (upper end) position of the face portion 12 is 0degrees relative to the center C of the hitting face, the foam material30 is provided inside the area of the face portion 12 that correspondsto the range between angle θ₁ (e.g., 20 degrees) and angle θ₂ (e.g., 60degrees). Note that the foam material 30 is not provided in portionsother than the hatched portions of the frame 10 (referred to hereinafteras non-hatched portions)

Also, as shown in FIG. 2, the width (length in the normal directionrelative to the hitting face) W₂ of the frame 10 in the hatched portionsis made greater than the width W₁ of the frame 10 in the non-hatchedportions. Specifically, the width W₁ is 20 mm, and the width W₂ is 22mm. Since the width of the frame 10 in the hatched portions is madegreater than that in the non-hatched portions in this way, more of thefoam material 30 can be provided in the frame 10, and the strength canbe increased even more.

The following describes the reason why the foam material 30 is providedto the frame 10 (the face portion 12) only partially in this way. As oneexample, top players (advanced players) tend to frequently use the areaon the tip side of the hitting face than the sweet spot S. For thisreason, there is a demand to increase particularly rigidity (strength)and vibration absorption in the areas of the frame 10 that correspond tothis area. Since the foam material 30 is provided in these areas in thepresent embodiment, it is possible to improve rigidity and vibrationabsorption when hitting a ball with the tip side of the hitting face.

Also, if the foam material 30 were uniformly provided in the entireinterior of the frame 10, the overall strength would increase, thusleading to a risk of a decrease in flexure, an increase in repulsiveforce, and a decline in ball control. Furthermore, the overall weight ofthe frame 10 would increase, thus making a reduction in weightdifficult. In contrast to this, with the present embodiment, the foammaterial 30 is provided on the two sides (the hatched portions) of theframe 10 (the face portion 12) that are on opposite sides of apredetermined area on the tip side than the sweet spot S, and the foammaterial 30 is not provided in the other areas (the non-hatchedportions), thus allowing the face portion 12 to flex to an appropriateamount when hitting a ball. This enables to improve control when hittinga ball. This also makes a reduction in weight possible.

Racket Manufacturing Method Present Embodiment

A method for manufacturing the tennis racket 1 of the present embodimentwill be described below with reference to FIGS. 3A to 3D. FIGS. 3A to 3Dare schematic diagrams showing the method for manufacturing the tennisracket 1.

The method for manufacturing the tennis racket 1 of the presentembodiment has a foam material arranging step, a sheet tube formingstep, a bending step, and a die molding step.

<Foam Material Arranging Step>

First, as shown in FIG. 3A, a carbon sheet 44 (which corresponds to aresin sheet) of prepreg fiber-reinforced resin (FRP) mainly made ofcarbon fiber is prepared, and the foam material 30 is arranged in areasthat correspond to the hatched portions in FIG. 1. The foam material 30is obtained by mixing a resin-based adhesive and a thermally expandablefoam material, and forming the mixture into a sheet shape.

Note that in FIG. 3A, the midpoint of the carbon sheet 44 in thelengthwise direction (left-right direction in the figure) will becomethe tip of the frame 10 (the face portion 12) of the tennis racket 1,and therefore positions on the frame 10 of the tennis racket 1 can bespecified in advance based on the distance from the midpoint. In thepresent embodiment, the size of the sheets of the foam material 30 andthe positions where they are arranged on the carbon sheet 44 are setsuch that the foam material 30 is provided to the hatched portions ofthe frame 10 in FIG. 1.

<Sheet Tube Forming Step>

Next, as shown in FIG. 3A, a tube 42 is fitted around a mandrel 40, andthe carbon sheet 44 having the foam material 30 arranged thereon iswound around the tube 42. Note that the mandrel 40 is a round bar-shapedmember (cored bar) whose length corresponds to the entire length of theframe 10 (the face portion 12, the shaft portion 13, and the gripportion 14) of the tennis racket 1. Also, the tube 42 is a flexibleresin tube (e.g., a nylon tube).

The mandrel 40 is then removed, and thus a cylindrical tube body(referred to hereinafter as a sheet tube 100) is formed as shown in FIG.3B.

FIG. 4A is a cross-sectional view taken along A-A in FIG. 3B, and FIG.4B is a cross-sectional view taken along B-B in FIG. 3B. As shown inFIG. 4A, in the A-A cross-section of the sheet tube 100, the foammaterial 30 is arranged on the outside of the tube 42. The carbon sheet44 is then wound on the outside of the foam material 30. In contrast, asshown in FIG. 4B, in the B-B cross-section of the sheet tube 100, thecarbon sheet 44 is wound on the outside of the tube 42, and the foammaterial 30 is not provided.

<Bending Step>

As shown in FIG. 3C, the sheet tube 100 is bent so as to conform to theshape of the frame 10 (the face portion 12, the shaft portion 13, andthe grip portion 14) of the tennis racket 1. Also, the yoke portion 16,which is formed using a configuration similar to that of the sheet tube100, is attached in the border portions between the face portion 12 andthe shaft portion 13, and a carbon sheet 18 is wound around the borderportions. Note that the foam material 30 maybe provided inside the yokeportion 16.

<Die Molding Step>

The sheet tube 100 bent in the shape of the frame 10 and the yokeportion 16 attached to the sheet tube 100 are set in a die 50 as shownin FIG. 3D. Heating is then performed, and air is introduced into thetube 42 through the opening at the end (base) of the grip portion 14portion of the sheet tube 100. The sheet tube 100 undergoes plasticdeformation due to being heated, and at this time, the carbon sheet 44is pushed and spread outward due to expansion of the tube 42, and thecarbon sheet 44 is pressed against the die 50. The plastically deformedsheet tube 100 is thus molded into a shape that conforms to the die 50.Note that although the foam material 30 expands (foams) at this time dueto being heated, this expansion is suppressed due to being subjected toair pressure from the tube 42. The foam material 30 then expands whenthe air pressure inside the tube 42 is released. Accordingly, in theareas (hatched portions) where the foam materials 30 are formed, thetube 42 is pressed into the surrounding foam material 30, and theinterior of the frame 10 is filled with the foam material 30. Thisresults in the molding of the frame 10 in which the carbon sheet 44 andthe foam material 30 are integrated.

Comparative Example

The following describes a method for manufacturing a tennis racketaccording to a comparative example. In this comparative example, aliquid-form foam material that has a high expansion ratio (e.g.,urethane) is injected into a heat-molded hollow frame through an openingat the base of the grip portion (end portion of the hollow portion), andthen is caused to undergo foaming. Due to such foam material (the foammaterial used in this comparative example being referred to hereinafteras the foam material 30′) undergoing foaming, the foam material 30′ isuniformly provided inside the entire the frame 10.

In contrast, with the present embodiment, the sheets of the foammaterial 30 are arranged on the carbon sheet 44 before the sheet tube100 is formed, thus enabling to provide the foam material 30 easily inarbitrary areas of the frame 10 by changing the size and arrangementposition of the foam material 30.

Foam Material Characteristics

The following describes a comparison of characteristics (e.g., hardness)of the foam material 30 according to the present embodiment and the foammaterial 30′ according to the comparative example. Note that asdescribed above, the foam material 30 is formed by mixing a resin-basedadhesive and a thermally expandable foam material, and the foam material30′ is a highly expandable urethane foam material.

The following experiments were performed on cases with the foammaterials alone and with them formed inside the frame 10. The results ofthese experiments are described below.

<Foam Material Hardness Test> Test Method

FIG. 5 is a schematic diagram showing aspects of a hardness testperformed on the foam material alone. Test pieces (samples) havingvertical and horizontal lengths of 20 mm and a height of 5 mm werecreated using the respective foam materials (alone) in the foamed state,and the amount of deformation when a force of 10 kgf (approximately 98N) was applied in the direction of the arrow shown in FIG. 5 wasmeasured.

Test Results

The amount of deformation was 2.44 mm with the foam material 30′,whereas the amount of deformation was 1.78 mm with the foam material 30.Accordingly, in the case of the foam material alone, it was confirmedthat hardness improved approximately 27% with the foam material 30 ofthe present embodiment compared to the foam material 30′ of thecomparative example.

<Test on Amount of Racket Cross-Section Deformation> Test Method

FIG. 6 is a schematic diagram showing aspects of the test on the amountof racket cross-section deformation.

The respective foam materials were provided inside the frame 10 (theface portion 12), and the amount of deformation when applying a force of50 kgf (approximately 490 N) toward the interior of the face portion 12was measured.

Test Results

The amount of deformation was 0.51 mm with the comparative example (thefoam material 30′), whereas the amount of deformation was 0.41 mm withthe present embodiment (the foam material 30). Accordingly, it wasconfirmed that an improvement of approximately 12% with respect to theamount of racket cross-section deformation was achieved with the foammaterial 30 of the present embodiment compared to the foam material 30′of the comparative example.

<Racket Bending Deformation Test> Test Method

FIG. 7 is a schematic diagram showing aspects of a racket bending test.

The areas where the respective foam materials were provided in theframes 10 (the face portions 12) were cut to a length of 100 mm, and theamount of bending was measured when supporting the two ends and applyinga force of 50 kgf (approximately 490 N) to the center.

Test Results

The amount of bending was 0.73 mm with the comparative example (the foammaterial 30′), whereas the amount of bending was 0.69 mm with thepresent embodiment (the foam material 30). Accordingly, it was confirmedthat an improvement of approximately 5% with respect to the amount ofracket bending deformation was achieved with the foam material 30 of thepresent embodiment compared to the foam material 30′ of the comparativeexample.

As described above, the tennis racket 1 of the present embodimentincludes the frame 10 that surrounds the hitting face for hitting aball, and the foam material 30 is provided inside (in hatched portions)the frame 10 on the two sides of the frame 10 that are on opposite sidesof an area of the hitting face on the tip side than the sweet spot S. Inthis way, the strength of the areas on the respective sides of the frame10 is increased. This enables to improve the rigidity and vibrationabsorption when hitting a ball with the tip side of the hitting face.Also, since the foam material 30 is not provided in the other areas (thenon-hatched portions), the face portion 12 can be allowed to flex by anappropriate amount when hitting a ball, and ball control can beimproved. It is also possible to achieve a reduction in weight comparedto the case of providing the foam material 30 in the entire frame 10.

Variation

The following describes a modified example of the present embodiment. Inthe above-described embodiment, the foam material was not provided inthe non-hatched areas (non-hatched portions) (the interior of the frame10 was hollow in the non-hatched portions) of the frame 10 of the tennisracket shown in FIG. 1. However, in this modified example, a foammaterial (foam material 32) different from the foam material 30 isprovided in the non-hatched portions.

The foam material 32 is a foam material (which corresponds to a foambody for filling) formed into a sheet shape using a material whosehardness is different from that of the foam material 30. Note that thehardness of the foam material 32 is lower than the hardness of the foammaterial 30.

FIG. 8 is an illustrative diagram of a method for manufacturing thevariation of the present embodiment.

As shown in FIG. 8, the foam material 30 is arranged on the carbon sheet44 at positions corresponding to the hatched portions, and the foammaterial 32 is arranged on the carbon sheet 44 at positionscorresponding to the non-hatched portions in FIG. 1. The followingmanufacturing method is similar to that of the above-describedembodiment. In this way, whereas the foam material was not provided inthe non-hatched portions of the frame 10 of the tennis racket 1 shown inFIG. 1 (the interior of the frame 10 was hollow in the non-hatchedportions) in the above-described embodiment, a foam material (the foammaterial 32) different from the foam material 30 is provided in thenon-hatched portions in this modified example. Since the foam material30 and the foam material 32 have different hardnesses, the strength canbe changed according to the position in the frame 10.

With this modified example, the rigidity of the hatched portions can beincreased, while the non-hatched portions are prevented from beinghollow. This enables to prevent chips, foreign particles, and the likefrom entering the frame 10 when the holes for the insertion of thestring are formed in the face portion 12, for example.

Note that although the foam material 32 is provided to all of thenon-hatched portions in this modified example, the modified example isnot limited to such. For example, the foam material 32 may be providedon only the tip side of the tennis racket 1 (the range between the twoareas where the foam material 30 is provided).

Also, the amount of foam material 30 used (the density thereof) in thehatched portions of the tennis racket 1 in FIG. 1 may be different fromthe amount of foam material 30 used (the density thereof) in thenon-hatched portions. For example, a configuration is possible in whicha thinly-formed sheet of the foam material 30 is arranged in thelengthwise direction of the carbon sheet 44, and then the foam material30 is further arranged thereon in areas corresponding to the hatchedportions (the positions where the foam material 30 is arranged in FIG.3A) . The strength can be changed according to the position in the frame10 in this case as well.

Other Embodiments

The above-described embodiment is for facilitating understanding of thepresent invention, and is not intended to limit the interpretation ofthe present invention. As a matter of course, the present invention canbe changed and modified without departing from spirit the invention, andequivalents thereof are encompassed in the present invention.

<Foam Material 30>

In the above-described embodiment, the foam material 30 is obtained bymixing a resin-based adhesive and a thermally expandable foam material,and forming the mixture into a sheet shape, but the present invention isnot limited to this. The foam material 30 may be configured from othercompositions as long as it can be formed into a sheet shape and expands(foams) due to heat or the like.

<Position where Foam Material 30 is Formed>

In the above-described embodiment, the foam material 30 was provided toareas on the two sides of the face portion 12 that are on opposite sidesof an area on the tip side than the sweet spot S (the areas of the faceportion 12 that correspond to the range from angle θ₁ to angle θ₂ inFIG. 1), but the present invention is not limited to such. For example,in a case such as when the sweet spot S is frequently used, the foammaterial 30 may be provided to the two sides of the face portion 12 thatare on opposite sides of the sweet spot S. Also, the area of the hittingface between the sweet spot S and the base side (side closer to the gripportion 14) is often used in the case of volleys performed mainly bymiddle-aged players, for example. In this case, the foam material 30 maybe provided in areas on the respective sides of the face portion 12 thatare on opposite sides of that area on the base side. Note that in thepresent embodiment, the areas where the foam material 30 is formed inthe frame 10 (the face portion 12) are determined by the positions wherethe foam material 30 is arranged on the carbon sheet 44, thus making itis possible to easily change the positions where the foam material 30 isformed.

What is claimed is:
 1. A tennis racket comprising: a frame thatsurrounds a hitting face for hitting a ball; wherein a foam material isprovided inside two side portions of the frame on opposite sides of apredetermined area of the hitting face, and the two side portions of theframe have a strength that is higher than a strength of another portionof the frame.
 2. A tennis racket according to claim 1, wherein a foammaterial for filling that is different from the foam material isprovided inside the another portion of the frame.
 3. A tennis racketaccording to claim 1, wherein the foam material is provided inside theanother portion of the frame, and an amount of the foam material in thetwo side portions is different from an amount of the foam material inthe another portion.
 4. A tennis racket according to claim 1, whereinthe frame has a frame portion that surrounds the hitting face on one endside, and has a grip portion on another end side, and the predeterminedarea is an area on the one end side than substantially a middle of thehitting face.
 5. A tennis racket according to claim 4, wherein when aposition of the one end of the frame portion is set at 0 degreesrelative to a center of the hitting face, the foam material is providedin a range of 20 degrees to 60 degrees of the frame portion.
 6. A methodfor manufacturing a tennis racket including a frame having a firststrength and a second strength that is higher than the first strength,comprising the steps of: arranging a foam material on a resin sheet inan area that corresponds to the second strength; forming a sheet tube bywrapping the resin sheet, on which the foam material is arranged, into acylindrical shape; and forming a frame in which the resin sheet and thefoam material are integrally molded, by bending the sheet cylinder to beset in a die, and then causing the foam material to undergo foaming.